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0d96ea1730
We see GTP-U originating from ports other than 2152 in the field. When
osmo-upf forwards these, we want to forward from our GTP-U port 2152,
since that is the only port osmo-upf has bound for GTP-U (for echo).
According to 3GPP TS 29.060, the *destination* port for GTP-U shall be
2152 -- but the source port is apparently allowed to be different.
Before this patch, we would forward GTP-U like this:
3.3.3.3:33333 -> (3.3.3.4:2152 UPF 2.2.2.2:33333) -> 1.1.1.1:2152
^^^^^
Instead we want to always send from UDP source port 2152:
3.3.3.3:33333 -> (3.3.3.4:2152 UPF 2.2.2.2:2152) -> 1.1.1.1:2152
^^^^
This hasn't shown up before because so far all GTP-U peers we saw
consistently used source port 2152.
Related: SYS#6773
Change-Id: Idaf43f1c2b915846b50a8b97305f0229e34ad539